Question

What is the resistance of \(\mathrm{P}-\mathrm{N}\) junction diode in forward biasing ? (A) zero (B) high (C) infinity (D) a few ohms

Short answer

The resistance of a PN junction diode in forward biasing is a few ohms (D).

Step-by-step solution

Understanding PN Junction Diode

A PN junction diode is formed by joining a P-type semiconductor material (having an excess of holes) with an N-type material (having an excess of electrons). The P-side is connected to a positive terminal, and the N-side is connected to a negative terminal. When the diode is forward biased, the voltage is applied in such a way that it decreases the barrier potential, allowing current to flow through the diode.

Diode Behavior in Forward Biasing

In the forward biasing condition, the electric field created by the battery works against the internal electric field of the PN junction diode, narrowing the depletion layer. This reduction in the depletion layer allows the majority charge carriers (holes and electrons) to pass through the junction relatively easily. This enables an electric current to flow through the circuit.

Analyzing the Resistance Options

In forward biasing, the diode allows the flow of current, thus, the resistance is not infinity. However, as it does not offer a completely unobstructed path, the resistance cannot be zero. Furthermore, the forward-biased diode provides relatively low resistance as it allows the current to flow easily. Now, let's analyze the given answer choices based on these observations. (A) zero - Not possible because it offers some resistance when the current flows through it. (B) high - Incorrect as the resistance in forward-biased diodes is not high. (C) infinity - Not possible, as in this case, no current would flow through it in forward biasing. (D) a few ohms - Appears to be the correct choice, as the forward-biased diode provides a relatively low resistance path for the current.

Conclusion

The resistance of a PN junction diode in forward biasing is a few ohms (D).

Key concepts

Forward Bias

Forward biasing in a PN junction diode refers to the condition where the external voltage is applied in a direction that reduces the potential barrier of the junction. This happens when the positive side of the voltage source is connected to the P-type material and the negative side to the N-type material. Doing so decreases the internal electric field across the junction.

This reduction in barrier potential means that the diode becomes conductive, allowing a significant amount of current to flow through it. The main reason for this behavior is the reduction in width of the depletion region which allows charge carriers—electrons and holes—to cross the junction more easily.

• Under forward bias, the external electric field opposes the internal field of the diode.
• Reduces the overall potential barrier, thus reducing resistance.
• Allows current to pass through effectively, meaning less energy is used to cross the barrier.

Forward biasing is a key operational mode for diodes in applications where conduction is required, such as rectifiers in power supplies.

Depletion Layer

The depletion layer in a PN junction diode is a region at the junction where mobile charge carriers are absent. Initially, when a P-type and an N-type semiconductor are joined, electrons from the N-type region diffuse into the P-type area and recombine with holes, creating a zone devoid of carriers—this is the depletion layer.

In forward bias, the depletion layer narrows. The external voltage applied reduces the height of the potential barrier. As a result, charge carriers need less energy to move across the junction, thus increasing the current flow.

• The depletion layer acts as a barrier to carrier movement in an unbiased or reverse-biased condition.
• Forward bias reduces this barrier, increasing current flow.
• Narrowing of the depletion layer signifies less resistance and easier charge carrier movement.

The behavior of the depletion layer is crucial for the function of diodes, dictating their conductive properties under different biasing conditions.

Semiconductor Materials

Semiconductor materials form the backbone of the PN junction diode. They are characterized by their ability to conduct electricity better than insulators, but not as well as conductors. This unique property is mainly due to their band structure, which allows them to carry an electric charge when energy, in the form of heat or light, is introduced.

The PN junction itself involves two types of semiconductor materials: P-type and N-type. P-type semiconductors are doped to create more holes than electrons, whereas N-type ones are doped to have excess electrons. When they are brought together to form a junction, they form the basis for the diode's operation.

• P-type materials have an excess of holes (positive charge carriers).
• N-type materials have an excess of electrons (negative charge carriers).
• The interaction between these materials at the junction creates the diode’s potential difference, critical for its switching properties.

Understanding these semiconductor materials is vital as they underpin how and why diodes function, including their ability to rectify currents and offer controlled resistance.